In various fields of communications, certain spectral requirements are imposed on transmitters due, for example, to regulatory limitations and/or interference considerations. In Home Phoneline networking (HomePNA), for example, these include: (1) FCC regulations, specifically part 15 (radiated emissions) and part 68 (conducted emissions), wherein the latter requires averaging the power over a period (time window) of 2 uS; (2) avoidance of audible noise in POTS (when apparent), which imposes peak constraints that seem to match the ones of part 68; and (3) avoidance of interference with HAM RF, wherein it is assumed that transmitting below −80 dBm/Hz in the HAM bands is sufficient.
Compatibility with FCC part 15 and 68 imposes constraints on the peak power, whereas the requirement to avoid interfering with HAM bands imposes requirements on the transmitted power in HAM bands (e.g. around 7 MHz). In the HomePNA2.0 specification, the foregoing requirements were met by the following solutions: using a PSD mask that complies with the FCC regulations and the HAM RF egress restrictions; normalizing the transmitted signal constellation according to the peak power (outermost symbols) to follow peak constraints; and using notch filters in the transmitter to comply with the power restrictions in the HAM bands. Normalizing the signal constellation according to the outermost symbols can cause a loss of up to 5 dB in large constellations. In the 256QAM constellation used in HPNA2.0, this imposes a loss of 4.23 dB. Using notch filters in the transmitter can result in a transmitted pulse that suffers from ISI and a longer impulse response, which in turn might degrade noise performance, and enhance error propagation in the receiver.
It is therefore desirable to provide spectral power management schemes that avoid undesired effects such as described above.
The invention attempts to avoid such undesired effects by implementing constellation shaping in the transmitter to support compliance with constraints on transmit power characteristics such as transmit power during a time window and/or transmit power in one or more predetermined frequency bands. Some embodiments of the invention provide preceding in the transmitter to avoid undesirable effects that can occur when special transmit filtering is used in the transmitter to comply with constraints such as constraints on transmit power in one or more frequency bands.